Conventional wavelength lockers monitor and control the wavelength of light produced by a light source. Conventional light sources are lasers, often distributed feedback Bragg reflective ("DFB") lasers. The laser is typically tuned to produce light of a predetermined wavelength, .lambda..sub.o. In reality, some portion of light produced by the laser does not have wavelength .lambda..sub.o. However, the distribution of wavelengths produced should be centered around .lambda..sub.o and not spread over a large range of wavelengths.
As the laser is used, the resonant characteristics of the cavity change. Consequently, the wavelength of the light produced by the laser may drift from the predetermined wavelength. In other words, the distribution of wavelengths produced may not be centered around .lambda..sub.o. Wavelength lockers monitor the wavelength of the light produced by the laser and control the laser to produce light centered around .lambda..sub.o.
In some wavelength lockers, wavelength of the incident light is Monitored using a diffraction grating. The diffraction grating is carefully aligned so that most of the light having wavelength .lambda..sub.o is transmitted in a particular direction. A small portion of light if same wavelengths is diffracted at an angle from the direction of propagation of light having the predetermined wavelength. The angle through which the small portion of light having wavelength .lambda..sub.o is diffracted depends on the design of the grating.
In such conventional wavelength lockers, a pair of filters are aligned at a predetermined angle from the direction of propagation of light having the predetermined wavelength. One filter is in the direction of propagation of light having a wavelength equal to .lambda..sub.o +.DELTA. where .DELTA. is a small change in the wavelength. The other filter is in the direction of propagation of light having a wavelength equal to .lambda..sub.o -.DELTA.. Each filter has a photodetector associated with it.
When the laser is producing light centered around the predetermined wavelength, .lambda..sub.o the intensity at the photodetectors is the same. When the light source is not producing Right centered around .lambda..sub.o, then the intensity of light at the photodetectors is different. The wavelength locker uses this information to adjust the characteristics of the laser, ensuring that light centered around the wavelength .lambda..sub.o is produced.
Although the conventional wavelength locker functions, a diffraction grating is typically expensive. In addition, each filter adds a cost to the system. Finally, the diffraction grating requires very precise alignment, making fabrication of the wavelength locker more difficult. Although a conventional wavelength locker may be made slightly less expensive by using a crystal in lieu of a diffraction grating, the crystal still adds a significant cost to the wavelength locker. In addition, a crystal must often be ordered from a vendor outside of the maker of the wavelength locker.
Accordingly, what is needed is a system and method for producing a wavelength locker which is cheaper and simpler to build. The present invention addresses such a need.